The invention primarily relates to the art of die casting such metals as aluminum, zinc, magnesium, copper, iron and their alloys and to a solution to a long standing problem therein; i.e., the lack of a commercially feasible die casting technique to produce castings having undercut regions. More specifically, this invention relates to a coating composition useful on expendable die casting cores and particularly in pressurized die casting methods.
1. Setting of the Invention
Traditional pressure die casting requires molds or dies which are able to withstand high temperatures and pressures. Ferrous materials are commonly used for such molds. Molds or cores of such materials for die castings having complex undercuts and reliefs are not possible since the cores for the undercut and relief cannot be removed from the casting. To attain such complex undercuts and reliefs, sand cores which were expendable and fragile have been employed at pressures under 30 psia, i.e., gravity fed casting.
Sand cores have been employed for high pressure die castings and have been composed of sand mixed with a binding agent. The mixture is formed into the desired core shape and cured, bound together, by use of heat or chemical reaction. The cured core can then be used in the casting process.
The major problem with such expendable core in high pressure die casting methods has been the inability of the single binder system to meet four core requirements. These are (a) good shake-out, (b) good washout resistance, (c) freedom from surface penetration, and (d) strength.
Good shake-out is necessary to facilitate core removal from the casting.
Washout resistance is the ability of the core to withstand erosion from the high metal velocities that occur during injection of the molten metal. The washed out sand adversely affects tolerances on the finished part, since the sand may become embedded within the casting.
Surface penetration or hardness is caused by the combination of high heat and pressure which breaks down the core surface and permits the metal to penetrate between the sand grains thus causing a sand/metal mixture interface at the surface of the casting. This condition is detrimental to subsequent machining and machine tool life. Moreover, should the entrained sand become separated from the casting surface after component installation, damage to related parts, such as the lubrication system of an automobile engine, could result.
The strength of the core is determined mainly by the sand binder used. Therefore, suitable coating and compositions must be compatible with binders having desired strength.
2. Prior Art
Although a wide variety of core and mold coatings have been developed for general foundry use, i.e., gravity fed casting, only a small number of coatings have been designed for the relatively harsh requirements connected with expendable high pressure die casting. Prior art coatings are illustrated by:
U.S. Pat. No. 4,096,293, issued June 20, 1978, entitled "Mold and Core Wash" and issued to Michael J. Skubon et al discloses a core wash useful in pressurized die casting methods made of a hydrocarbon solvent, fumaric resin, particulated calcium aluminate and a suspending agent. The composition comprises 5 to 90 wt % solvent, 0.5 to 5 wt % resin, 5 to 80 wt % calcium aluminate and 0.1 to 2 wt % suspending agent. The fumaric resin is described as the reaction product of fumaric acid, gum rosin and pentaerythritol. The suspending agents are disclosed to include high molecular weight polymers, polyacrylates, colloidal silicas, clay, vegetable gums, and amine-treated bentonite. Wetting agents are disclosed by U.S. Pat. No. 4,096,293 to include methyl alcohol, water, and anionic and cationic surfactants.
The prior art is further illustrated by U.S. Pat. No. 4,001,468, issued Jan. 4, 1977, entitled Method for Coating Sand Cores and Sand Molds and issued to M. J. Skubon et al which discloses a wash coating useful for preventing core erosion. The composition of the coating includes an organic vehicle, suspending agent, refractory material and an organic polymer or copolymer. The organic vehicle is described as having a kauri-butanol value (ASTM D1133) of 36 or higher. The suspending agent is described as including clay, vegetable gums, and amine-treated bentonite in ratio of suspending agent to vehicle of between 1:80 and 1:250. The refractory powder is described as including graphite, coke, mica, silica, aluminum oxide, magnesium oxide, talc, and zircon flour in a weight ratio of refractory to vehicle of between 1:2.5 and 1:3.5. The organic polymer or copolymer can be vinyl toluene butadiene polymer, styrene/butadiene copolymer, vinyl toluene/acrylate copolymer, styrene/acetylene copolymer, acrylate homopolymer, and styrene/butadiene copolymer in weight ratios of polymer/copolymer to vehicle between 1:50 and 1:200.
Disadvantages of Skubon include use of an organic binder which when heated releases an outgas which causes pores in the casting and lost strength. Further, the Skubon coatings are typically powdery and have reduced scratch resistance or hardness. Hardness is an indication of resistance to metal penetration or metal burning and erosion.